Material properties that determine electromagnetic (EM) or elastic (EL) wave propagation and scattering in the materials often show a variation with the field strength in the waves. Such materials are termed non-linear and give rise to nonlinear wave propagation and nonlinear scattering of both EM and EL waves. Measurements or imaging of nonlinear scattering sources are in many situations useful to identify properties of such materials.
Both the forward wave propagation and local scattering of EM or EL waves have mathematical similarities, and methods and instrumentation for imaging therefore have similar structures. Examples of uses of EL waves are material testing both with shear waves and compression waves, ultrasound medical imaging with compression waves, SONAR sub-sea and geological measurements, and seismic imaging. EM waves have similar uses, where particularly new developments of EM technology appear useful for both geological and medical imaging, providing added information to elastic wave images. EM imaging in the infra-red and optical frequency ranges also provides useful information both for material testing and medical imaging.
The nonlinear scattering can for EM or EL waves be separated into a parametric and a resonant scattering type. For EL waves, the parametric scattering originates from a nonlinear variation of the local elasticity parameters with the amplitude of the local elastic wave field, where spatial variations of the nonlinear variation produce the nonlinear scattering. For EM waves, the parametric scattering originates from a nonlinear variation of the local dielectric constant or magnetic permeability with the amplitude of the local EM wave field, where spatial variations of the nonlinear variation produce the nonlinear scattering. With elastic compression waves, referred to as acoustic waves, one for example gets strong nonlinear parametric scattering at the interface between soft materials and hard materials, for example as found with ultrasound nonlinear scattering from micro calcifications in soft tissue or acoustic scattering from hard objects in soil like mines or other objects. One also gets strong nonlinear scattering at the interface between harder materials and much softer materials, for example as found with ultrasound scattering from gas micro-bubbles in blood or gas filled swim-bladders of fish and the like in water.
Resonant nonlinear scattering has a time lag involved, which in some situations can be used to separate signal components from local nonlinear scattering and forward propagation distortion of the incident waves. However, the current invention provides further advantages for imaging of local resonant nonlinear scattering sources.
For acoustic waves, gas micro-bubbles show resonant scattering, for example, where the resonance originates from the energy exchange between the nonlinear elasticity of the bubble with shell and gas, and a co-oscillating fluid mass around the bubble with a volume approximately 3 times the bubble volume. As both the elasticity and the mass vary with bubble compression, the resonance frequency is nonlinearly affected by the incident acoustic wave field, producing a particularly strong nonlinear scattering with a large amount of harmonic components of the incident frequency (n-times the incident frequency) and even sub-harmonic components of the incident frequency (a fraction of the incident frequency) in the scattered field, and supra-harmonic components (bands around the harmonic components) of the incident frequency. However, for imaging at frequencies well above the bubble resonance frequency, the nonlinear scattering is much lower, and the present invention provides solutions for enhanced imaging of micro-bubbles at frequencies above the resonance frequency.
Resonant nonlinear EM scattering originates in the interaction between the wavefield and the atoms and molecules, which is best described within the realm of quantum physics. An example of EM resonant scattering is fluorescence which has similarities to sub-harmonic acoustic scattering. When for example the incident frequency is in the ultraviolet range, the scattered frequency can be in the visible range. The scattered frequency can also be the same as the incident frequency which is termed “resonant fluorescence”. Another example is two-photon quantum scattering that is similar to 2nd harmonic parametric scattering, but includes detailed atomic dynamics with time lags in the process.
There is also found a nonlinear interaction between EM and EL waves in materials, where for example EL compression waves change the EM material parameters in the process called the acousto-optic effect. Absorption of EM waves in materials produces a rapid, local heating of the material that generates acoustic waves in a process called the photo-acoustic effect. The invention hence addresses both EM and EL waves, and combinations of these, where the waves referred to in the description and claims can be both EM and/or EL waves.
With a single frequency band incident wave, the parametric nonlinear scattering produces harmonic components of the incident frequency band in the scattered wave. With dual band incident waves that interact locally, the parametric nonlinear scattering produces bands around convolutions of the incident frequency bands, resulting in bands around sums and differences of the incident frequencies. However, the nonlinear variation of the material parameters also produces an accumulative nonlinear distortion of the forward propagating wave. When the pulse length of the high frequency pulse increases above approximately a period of the low frequency pulse, the linear scattering from the nonlinear forward propagation distortion has a similar signature to the local nonlinear scattering, and it is in this case difficult to distinguish the signal components that arises from linear scattering of the nonlinear propagation distortion of the incident wave, and the signal components that occur from local nonlinear scattering. This is for example the situation with current harmonic imaging with medical ultrasound imaging.
On the contrary, when the pulse length of the high frequency pulse becomes shorter than approximately a half period of the low frequency pulse, it is possible to highly suppress the linear scattering components to measure or image the nonlinear scattering components, for example as shown in U.S. Pat. No. 8,038,616 and U.S. patent application Ser. Nos. 12/351,766, 12/500,518, and 13/213,965. Multiple scattering of EL and EM waves from strong scatterers often produce strongly disturbing noise in the measurements and images. When the pulse length of the high frequency pulse becomes shorter than half the wave length of the low frequency pulse, it is possible to highly suppress this multiple scattering noise, where some methods are presented in the cited US patent applications. Based on this background, the object of the current invention is to present improved methods and instrumentation for measurement and imaging of nonlinear scattering components and suppression of the multiple scattering noise, both with elastic and electromagnetic waves.